Many transistors formed over a glass substrate or the like are manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in liquid crystal display devices. Although transistors manufactured using amorphous silicon have low field-effect mobility, they can be formed over a larger glass substrate. On the other hand, although transistors manufactured using polycrystalline silicon have high field-effect mobility, they are not suitable for being formed over a larger glass substrate.
Other than the transistors manufactured using silicon, in recent years, a technology in which a transistor is manufactured using an oxide semiconductor and applied to an electronic device or an optical device has attracted attention. For example, a technology in which a transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide as an oxide semiconductor and used for a switching element of a pixel of a display device or the like is disclosed in Patent Document 1 and Patent Document 2.